Exhaust gases ventilated from motor tunnels, underground parking, and indoor spaces equipped with apparatus which produces harmful gases, are largely different from flue gases ventilated from incineration apparatus in that exhaust gases have normal temperature and low NO.sub.x concentration, and the NO.sub.x concentration drastically varies in an instant. Flue gases have been treated by a selective catalytic reduction process (NH.sub.3 reductive denitration process) for removing NO.sub.x therefrom. In the selective catalytic reduction process, NO.sub.x is reduced to nitrogen (N.sub.2) using ammonia (NH.sub.3) as a reducing agent with a presence of titania (TiO.sub.2) as a catalyst. However, the selective catalytic reduction process is not directly applicable for removing NO.sub.x from exhaust gases. For this reason, it has been considered that for the exhaust gas, after the NO.sub.x concentration is increased by passing the exhaust gas through an appropriate NO.sub.x adsorbent to adsorb NO.sub.x and heating and desorbing the adsorbent, the aforementioned NH.sub.3 reductive denitration process is adopted. The following adsorbents have been known as NO.sub.x adsorbents used for the process.
1) Activated alumina (Al.sub.2 O.sub.3) type NO.sub.x adsorbent (disclosed in Japanese Unexamined Patent Publication No. 4-367707). This adsorbent includes oxides of sodium and calcium, and in addition thereto oxides of manganese, iron or copper, thereby having an enhanced gas adsorption ability.
2) Low-concentration NO.sub.x adsorbent (disclosed in Japanese Unexamined Patent Publication No. 5-123568). This adsorbent includes anatase-type TiO.sub.2, used as a carrier, and ruthenium supported on the anatase-type TiO.sub.2.
3) An adsorbent for NO.sub.x, and especially NO.sub.2 (disclosed in Japanese Unexamined Patent Publication No. 7-88363). This adsorbent includes a carrier, and at least one precious metal selected from the group consisting of platinum, gold, ruthenium, rhodium, and palladium or a compound thereof, and if necessary, a metallic oxide of metals such as manganese, iron, cobalt, nickel, copper, and zinc supported on the carrier.
4) An NO.sub.x oxidizing and adsorbing agent (disclosed in Japanese Unexamined Patent Publication No. 8-173796). This agent contains .gamma.-MnO.sub.2-x (where 0.ltoreq..times..ltoreq.0.1) having a specific surface area of 100 m.sup.2 /g or more as a main component.
On the other hand, examples of known catalysts conventionally used for oxidizing CO into CO.sub.2 at normal temperature include a metallic oxide containing a copper oxide and a manganese oxide in a mixed state as main components (hopcalite catalyst), activated charcoal including platinum or palladium, and alumina. Also known as catalysts are a mixture of palladium or a compound thereof and activated manganese dioxide (Japanese Examined Patent Publication No. 63-33419), and a catalyst which includes alumina, and palladium salt, copper salt, and nickel salt supported on the alumina (Japanese Unexamined Patent Publication No. 60-7942).
Examples of known O.sub.3 removing agent capable of removing O.sub.3 at normal temperature include a catalyst containing a manganese oxide or a nickel oxide and an aluminum oxide as active ingredients (Japanese Unexamined Patent Publication No. 6-154601), a manganese dioxide catalyst (Japanese Unexamined Patent Publication No. 7-246335), a catalyst which includes a composite silica.multidot.boria.multidot.alumina, amorphous manganese oxide and amorphous palladium oxide supported on the compound oxide, and preferably further containing oxide of silver, iridium, rare-earth metals and/or transition metals (Japanese Unexamined Patent Publication No. 8-10619).
Examples of known catalyst capable of simultaneously removing CO and O.sub.3 at normal temperature include a molded product containing transition metal such as iron, nickel, copper and manganese or oxides thereof as catalytic activation ingredients (Japanese Examined Patent Publication No. 6-26671).
However, the aforementioned conventional NO.sub.x adsorbents 1) to 3) have the following problems.
Although the adsorbent 1) is intended for use in adsorbing NO.sub.x, it actually is capable of adsorbing NO.sub.2 only. In many cases, NO forms the most part of the total NO.sub.x concentration and NO.sub.2 forms only several to 10 percent. Therefore, prior to treatment, it is necessary to add ozone (O.sub.3) to the exhaust gas being treated to oxidize NO into NO.sub.2. If the amount of added O.sub.3 is not sufficient, unoxidized NO remains without being adsorbed. Contrary to this, if the amount of added O.sub.3 is too much, the excessive O.sub.3, which does not react with NO, remains as harmful gas. There is also a fear that O.sub.3 may leak from the exhaust gas treatment apparatus, regardless of the amount of O.sub.3 added. In addition, the adsorbent 1) after adsorbing NO.sub.x requires to be heated at temperature as high as 450.degree. C. in order to desorb NO.sub.x.
Although the adsorbent 2) is intended for used in adsorbing NO.sub.x, there is no clear description as to the adsorbing abilities of NO and NO.sub.2 respectively. Therefore, its ability of adsorbing NO is not definitive. In addition, the adsorbent 2) after adsorbing NO.sub.x also requires to be heated at temperature as high as 350.degree. C. in order to desorb NO.sub.x.
Although the adsorbent 3) is intended for use in adsorbing NO.sub.x. However, its ability of adsorbing NO is remarkably poorer than that of adsorbing NO.sub.2 (and its ability of adsorbing NO lasts for only a short time). Therefore, the adsorbent 3) requires to add O.sub.3 to the exhaust gas to oxidize NO into NO.sub.2, as is the case of the adsorbent 1),
It is required to oxidize NO into NO.sub.2 by adding O.sub.3 to the exhaust gas; otherwise, NO.sub.x cannot be adsorbed sufficiently. In most cases, exhaust gases have an NO.sub.2 concentration lower than NO concentration. However, if a large amount of NO in the exhaust gas being treated is oxidized into NO.sub.2, there is a fear that only some part of NO.sub.2 is adsorbed by the adsorbent, while the rest NO.sub.2 is not adsorbed by an adsorbent, and the ventilated exhaust gas may have the NO.sub.2 concentration remaining unchanged or conversely increased exceeding the value regulated by the Environmental quality standard. Due to such disadvantages, the adsorbents 1) to 3) are not preferable..
Furthermore, in many cases, exhaust gases ventilated from automobile tunnels and underground parking also contain sulfur oxide (SO.sub.x) such as sulfur dioxide (SO.sub.2) and sulfur trioxide (SO.sub.3). The concentration of SO.sub.x is generally lower than that of NO.sub.x. However, SO.sub.x generally reacts with various kinds of metals to produce accumulative sulfur compound (such as sulfate), and the adsorbents 1), 2), and 3) contains metals as main components easy to react with sulfur to produce sulfur compound, such as heavy metal oxide, activated alumina, and ruthenium compound. Thus-produced sulfur compound may cause deterioration of the harmful gas adsorbing ability of the adsorbents (i.e., the adsorbents are poisoned by the sulfur compound).
In the case of employing the adsorbent 4), a large part of NO is oxidized into NO.sub.2 which is then discharged. The adsorbent (4) adsorbs only a part of NO. Therefore, it is necessary to additionally provide an NO.sub.2 adsorbent on the downstream of the adsorbent (4). Accordingly, as is the cases of employing the adsorbents 1) and 3), NO.sub.x cannot be sufficiently removed unless remarkably enhanced NO.sub.x adsorbing ability is given to the adsorbent 4).
Furthermore, the hopcalite catalyst, known as a catalyst used for removing CO, exhibits CO removing activity at normal temperature only for a short time, especially in the presence of humidity. Therefore, hopcalite catalyst requires anti-humidity treatment when stored and a drying treatment prior to be used. In addition, a catalyst having a carrier supporting metals such as platinum and palladium is easily poisoned by CO, because platinum and palladium powerfully adsorb CO at normal temperature. In order to avoid this problem, such a catalyst is heated at a temperature of 100.degree. C. or higher, or is subjected to treatment for increasing platinum or palladium concentration to several mass percent. However, there treatments require high cost.
Japanese Examined Patent Publication No. 63-33419 discloses a catalyst including active manganese dioxide. With active manganese dioxide, the catalyst has higher CO removing ability than the catalyst including an inactivate carrier and a platinum compound or a palladium compound supported on the carrier. However, its ability of removing CO is still insufficient at normal temperature, especially in the presence of moisture.
Japanese Unexamined Patent Publication No. 60-7942 discloses a catalyst used for removing CO. However, this catalyst is applicable to an exhaust gas containing CO having a concentration of about 100 ppm or lower. This is because palladium salt, contained in the catalyst, is reduced to a metallic palladium in the presence of CO having high concentration. In addition, the catalyst requires humidity in order to keep its sufficient CO removing ability.
Japanese Unexamined Patent Publication Nos. 6-154601, 7-246335, and 8-10619 respectively disclose a catalyst used for removing O.sub.3. The catalyst has a problem that its O.sub.3 removing activity lasts only for a very short time especially at high humidity. Japanese Examined Patent Publication No. 6-26671 discloses a catalyst used for removing CO and O.sub.3. Although this catalyst is capable of removing both CO and O.sub.3, the removing activity lasts only for a short time especially at high humidity, and this tendency is remarkable for CO removal.